This disclosure relates generally to tracking systems that use magnetic fields to determine positions and orientations of an object, such as systems used for tracking instruments and devices during surgical interventions and other medical procedures. More particularly, this disclosure relates to a system and method for reducing magnetic field distortion in such systems.
Tracking systems have been used in various industries and applications to provide position information relating to objects. For example, electromagnetic tracking may be useful in aviation applications, motion sensing applications, and medical applications. In medical applications, tracking systems have been used to provide an operator (e.g., a physician) with information to assist in the precise and rapid positioning of a medical device located in or near a patient's body. In general, an image may be displayed on a monitor to provide positioning information to an operator. The image may include a visualization of the patient's anatomy with an icon on the image representing the device. As the device is positioned with respect to the patient's body, the displayed image is updated to reflect the correct device coordinates. The base image of the patient's anatomy may be generated either prior to, or during, the medical procedure. For example, any suitable medical imaging technique, such as X-ray, computed tomography (CT), magnetic resonance imaging (MRI), positron emission tomography (PET), and ultrasound, may be utilized to provide the base image displayed during tracking. The combination of the base image and the representation of the tracked device provides positioning information that allows a medical practitioner to manipulate a device to a desired position and/or associate information gathered to a precise location.
To determine device location, tracking systems may utilize a method of electromagnetic (EM) field generation and detection. Using this method, at least one magnetic field is generated from one or more EM sensors, and the magnetic fields are detected by one or more complementary EM sensors. In such a system the mutual inductance of the EM field detected may be processed to resolve a position and/or orientation of the EM sensors relative to one another. For example, an EM sensor may be fixed in a known position, with a complementary EM sensor mounted at the operative end of a device. While the EM sensor generates a magnetic field, the magnetic field characteristics may be detected by the complementary EM sensor. The detected characteristics may be processed to determine the position and orientation (e.g., the X, Y and Z coordinates, as well as the roll, pitch and yaw angles) of the EM sensors relative to one another.
However, as will be appreciated, the presence of field distorting objects in or near the magnetic field may cause distortions of the magnetic field emitted from the EM sensors. As a result, the magnitude and direction of the magnetic field sensed by the complementary EM sensor may be inaccurate. Distortions, such as these, may come from a multitude of sources, including: signals from other electromagnetic sources, the magnetic fields generated by eddy currents in another conductive object, and the field distorting effect of a ferro-magnetic objects. Unless compensated for, or significantly reduced, these distortions and inaccuracies may produce an error in the determined location of the device. For example, a source of magnetic field distortion may include the equipment surrounding the tracking system (e.g., a metal surgery table or conductive medical devices). In these instances, the electromagnetic field generated by the EM sensors may induce eddy currents into a metal surface. The eddy currents may produce additional electromagnetic fields that distort the electromagnetic field originally generated by the EM sensor, thereby creating errors in the determined position and orientation of the complementary EM sensor. Although, methods are known to map and compensate for the distortions, if the distortions become too significant, mapping may not be capable of compensating for the distortions.
Accordingly, there is a desire to provide an electromagnetic field tracking system where the effects of electromagnetic field distorters is reduced and/or eliminated.